The invention relates to a hydraulic control apparatus for an automatic transmission installed in a vehicle or the like, for example, and more particularly to a hydraulic control apparatus for an automatic transmission in which a secondary pressure from a second pressure regulating valve is supplied to a torque converter and a lock-up clutch.
Typically, an automatic transmission installed in a vehicle or the like, for example, is provided with a hydraulic transmission apparatus for hydraulically transmitting the output of an engine to an input shaft of a speed change mechanism. In other words, the hydraulic transmission apparatus is provided with a torque converter which is capable of permitting a rotation speed difference between an output shaft (crankshaft) of the engine and the input shaft of the speed change mechanism. In recent years, to achieve reductions in fuel consumption and so on, more and more hydraulic transmission apparatuses for automatic transmissions are being provided with a lock-up clutch that is capable of directly coupling (locking up) the output shaft of the engine and the input shaft of the speed change mechanism.
Incidentally, an automatic transmission is provided with a clutch and a brake for forming a power transmission path of a transmission gear mechanism, and a hydraulic control apparatus for controlling the engagement and disengagement of the clutch and brake. The hydraulic control apparatus is provided with a solenoid valve which outputs a control pressure in accordance with a throttle opening, and a primary regulator valve which is controlled on the basis of the control pressure. A line pressure corresponding to the throttle opening is regulated by the solenoid valve and primary regulator valve, and the line pressure is supplied to a hydraulic servo or the like, of the clutch and brake. Meanwhile, to achieve an improvement in the durability of the torque converter in particular, a secondary regulator valve, which is likewise controlled on the basis of the control pressure, generates a secondary pressure obtained by reducing the line pressure, and supplies the torque converter and lock-up clutch with this secondary pressure.
However, in cases such as when the engine output is great, for example, and the required transfer torque capacity of the lock-up clutch is large, the secondary pressure must be raised, or in other words, the line pressure is unnecessarily increased, and this may adversely affect the fuel economy and so on. To solve this problem, it has been proposed that the control pressure from the aforementioned solenoid valve be input into the two oil chambers of the secondary regulator valve simultaneously, thereby raising the gain of the secondary regulator valve so as to prevent unnecessary increases in the line pressure (see Japanese Unexamined Patent Application Publication 2003-42287, for example).
A brief description of an example of a conventional hydraulic control apparatus for an automatic transmission will now be provided. FIG. 4 is a view showing an example of a conventional hydraulic control apparatus for an automatic transmission.
The automatic transmission comprises a hydraulic transmission apparatus 4 having a torque converter 2 and a lock-up clutch 3. As shown in FIG. 4, a hydraulic control apparatus 50 for the automatic transmission is constituted by a strainer 5, an oil pump 6, a linear solenoid valve SLT, a solenoid valve S1, a primary regulator valve 7, a secondary regulator valve 58, a lock-up relay valve 59, a lock-up control valve 10, a check valve 12, an orifice 19, an oil cooler 30, and a lubricating oil passage (LUBE) 31.
When the oil pump 6 is driven by the driving force of an engine, not shown in the drawing, for example, oil is aspirated from an oil pan, not shown in the drawing, via the strainer 5 such that oil pressure is supplied to the primary regulator valve 7 via an oil passage a1. Further, a modulator pressure PMOD is input into the linear solenoid valve SLT from a modulator valve, not shown-in the drawing, and on the basis of the throttle opening, the linear solenoid valve SLT outputs a control pressure PSLT to oil passages c1, c2, c3, c4 through an output port SLTb. A spool position of the primary regulator valve 7 is then adjusted by the urging force of a spring 7s, the control pressure PSLT input into an oil chamber 7a thereof via the oil passage c2, and a feedback pressure input into an oil chamber 7b thereof, whereby the primary regulator valve 7 regulates the oil pressure in oil passages a1, a2, a3, a4 to a line pressure PL corresponding to the throttle opening while regulating the oil pressure that is returned to the oil pump 6 via an oil passage d1.
Meanwhile, the secondary regulator valve 58 is structured such that a spool 58p, a spring 58s, a plunger 58j, and a lid portion 58g having a sleeve portion 58i are inserted into a valve hole 22 formed in a valve body 20 in the direction of an arrow X, and the lid portion 58g is fixed to the valve body 20 by a key 58h, thereby forming a first oil chamber 58a, a second oil chamber 58b, and a third oil chamber 58c. A land portion 58pl1 of the spool 58p and the plunger 58j are formed with an outer diameter D3, while a land portion 58pl2, a land portion 58pl3, and a land portion 58pl4 are formed with an outer diameter D4.
When the lock-up clutch 3 is set in a state of release, the solenoid valve S1 is controlled such that a signal pressure PS1 is not output, and thus a spool 59p of the lock-up relay valve 59 is set in a position on the left half of the drawing, whereby the control pressure PSLT supplied by the linear solenoid valve SLT through the oil passage c4 is blocked by the lock-up relay valve 59. As a result, the secondary regulator valve 58 receives only the control pressure PSLT, which is input into the first oil chamber 58a via the oil passage c3, and the feedback pressure, which is input into the second oil chamber 58b. 
The position of the spool 58p of the secondary regulator valve 58 is then adjusted by the urging force of a spring 58s, the control pressure PSLT input into the first oil chamber 58a, and the feedback pressure input into the second oil chamber 58b, whereby the secondary regulator valve 58 regulates the oil pressure in oil passages b1, b2, b3, b4, b5, b6, which are connected to a port 58f, in accordance with the throttle opening to a secondary pressure PSEC that is much lower than the line pressure PL while regulating the oil pressure that is returned to the oil pump 6 through a port 58d via an oil passage d2.
Note that the secondary pressure in this state is regulated to a lower pressure than the secondary pressure when the lock-up clutch 3 is engaged, as will be described below, and is therefore referred to as a low secondary pressure PSECLo. Further, assuming that the pressure receiving area of the outer diameter D3 is A3 (i.e. D32π/4) and the urging force of the spring 58s is FSP, the low secondary pressure PSECLo may be expressed by the formula A3*PSECLo=A3*PSLT+FSP, whereby PSECLo=PSLT+FSP/A3, and therefore the gain of the secondary regulator valve 58 (the input/output ratio of the secondary pressure PSEC to the control pressure PSLT) is 1.
The low secondary pressure PSECLo is supplied to the torque converter 2 through the oil passage b5 via the lock-up relay valve 59. Since the low secondary pressure PSECLo, which is much lower than the line pressure PL, is supplied to the torque converter 2, high pressure is not applied to the torque converter 2, leading to an improvement in its durability. Furthermore, the low secondary pressure PSECLo is regulated according to the throttle opening, and hence when the engine output rises in accordance with the throttle opening, the torque transfer capacity of the torque converter 2 also rises or increases, enabling normal torque transfer.
When the lock-up clutch 3 is engaged, on the other hand, the solenoid valve S1 is controlled such that the signal pressure PS1 is output, and thus the spool 59p of the lock-up relay valve 59 is set to a position as indicated by the right half position of the lock-up relay valve 59 in FIG. 4 of the drawings. As a result, a port 59l and a port 59m of the lock-up relay valve 59 communicate with each other such that the control pressure PSLT supplied from the linear solenoid valve SLT through the oil passage c4 is input into the third oil chamber 58c of the secondary regulator valve 58 via an oil passage t1. Thus the secondary regulator valve 58 receives the control pressure PSLT, which is input into the first oil chamber 58a, the feedback pressure, which is input into the second oil chamber 58b, and the control pressure PSLT, which is input into the third oil chamber 58c. 
The control pressure PSLT input into the third oil chamber 58c acts to depress the spool 58p in a direction, as indicated by the arrow X in FIG. 4, in accordance with the difference between the pressure receiving areas of the plunger 58j having the outer diameter D3 and the land portion 58pl4 having the outer diameter D4. As a result, the oil pressure that is returned to the oil pump 6 through the port 58d via the oil passage d2 is throttled, and the oil pressure in the oil passages b1, b2, b3, b4, b5, b6 connected to the port 58f is regulated in accordance with the throttle opening to a higher secondary pressure (“high secondary pressure” hereafter) PSECHi than the low secondary pressure PSECLo.
Assuming that the pressure receiving area of the outer diameter D3 is A3 (i.e. D32π/4), the pressure receiving area of the outer diameter D4 is A4 (i.e. D42π/4), and the urging force of the spring 58s is FSP, the high secondary pressure PSECHi may be expressed by the formula A3*PSECHi=A3*PSLT+(A4−A3)*PSLT+FSP, whereby PSECHi=A4/A3*PSLT+FSP/A3, and therefore the gain (input/output ratio) of the secondary regulator valve 58 is A4/A3 (A4>A3), or in other words, the gain is greater than 1.
The high secondary pressure PSECHi is supplied to the lock-up clutch 3 through the oil passage b6 via the lock-up control valve 10 and the lock-up relay valve 59. Further, the high secondary pressure PSECHi supplied through the oil passage b5 is reduced by the orifice 19 and check valve 12 and then supplied to the torque converter 2. As a result, the lock-up clutch 3 is engaged by the differential pressure between the pressure supplied to the lock-up clutch 3 and the pressure supplied to the torque converter 2.
By increasing the gain of the secondary regulator valve 58 beyond or above 1 in this manner, the secondary pressure PSEC can be raised or increased beyond the input control pressure PSLT, and the required transfer torque capacity can be secured in the lock-up clutch 3 without raising the control pressure PSLT of the linear solenoid valve SLT unnecessarily, or in other words without raising or increasing the line pressure PL unnecessarily, enabling an improvement in fuel economy.
However, when control pressure is input into the two oil chambers 58a, 58b of the secondary regulator valve 58 in the manner described above, the land portion 58pl4 positioned in the intermediate part of the spool 58p must be formed with a greater diameter than the plunger 58j (i.e. D4>D3) in order to cause the control pressure PSLT input into the third oil chamber 58c in particular to act so that the gain increases beyond or above 1.
Further, if the land portion 58pl1 of the spool 58p is formed with an identical diameter to the land portion 58pl4, the gain of the low secondary pressure PSECLo in particular decreases (such that PSECLo=A3/A4*PSLT+FSP/A4), and hence when the throttle opening increases, the torque transfer capacity of the torque converter 2 may be insufficient in relation to the increase in engine output.
Typically, the various valves of the hydraulic control apparatus are structured by forming a valve hole in a plate-form valve body, inserting a spool, and sealing in the spool with a lid-form member. However, when the land portion 58pl4 positioned in the intermediate part of the spool 58p has a large diameter as described above, a gap is formed between the part positioned on the rear end side (in the insertion direction) of the inserted spool 58p and the valve hole 22, and hence a member such as the aforementioned sleeve 58i must be provided. Further, when the sleeve 58i is provided between the valve hole 22 and spool 58p, for example, the axial center of the valve hole 22 and the axial center of the sleeve 58p are not always concentric, and if the spool 58p is simply provided in an extending manner, the resulting eccentricity may lead to problems such as an increase in contact resistance between the spool 58p and sleeve 58p or the formation of a gap therebetween. To avoid these problems, the plunger 58j must be provided to slide concentrically in relation to the sleeve 58i and the sleeve 58p must be depressed via the plunger 58j. 